Goals
Understand what forEach does
Write our own version of the forEach function
Why Do I Need To Know This?
Write cleaner and more declarative code
Use built in higher order functions and callback functions
Foundation for modern libraries and frameworks
forEach
Loops through an array
Runs a callback function for each value in the array and then returns undefined
forEach will always return undefined - no matter what
An Example
let arr = [1,2,3];
arr.forEach(function(value, index, array){
console.log(value);
});
// 1
// 2
// 3
// undefined
JavaScript
Let’s Build Our Own!
Loops through an array
Runs a callback function on each value in the array
returns undefined
function forEach(array, callback){
for(let i = 0; i < array.length; i++){
callback(array[i], i, array);
}
}
JavaScript
An Example
function countZeroes(arr){
let total = 0;
arr.forEach(function(val){
if(val === 0) {
total++;
}
});
return total;
}
countZeroes([2,4,6]); // 0
countZeroes([0,1,2,0,1]); // 2
JavaScript
Remember - forEach always returns undefined!
When You Would Use forEach
You want to iterate over an array, but the return value of your callback is not important
Almost all of the time there are better options…
Recap
forEach iterates over an array, runs a callback on each value and
forEach returns undefined